Peugeot 308 (2010).with heater problems. Only warm air from vents & foot well. Coolant leaking too but engine NOT over heating. No visible leaks

Car heater works but only blows warm-ish air on small vents and footwell blowers. Windscreen vents not blowing warm air like other vents and temp seems to drop or not even be there on windscreen vents. Have to start heater on either vents or foot well blowers or else air is colder. Can also hear difference in fan blowing faster/slower whilst car is moving (without adjusting by using fan controls) and it seems to vary and have a mind of its own!!! Not the best of heaters by far and even on No5 does not seem that great! In fact warm-ish air gets cooler when heater turned up!
Have been losing coolant too but cant see any obvious leaks under car. Radiator has been changed and system drained and refilled but still have the problem with heater. Once engine is warm, Pipes from coolant/to radiator are hot and also hot from radiator. I checked pipes going into dashboard from engine from under the passenger footwell (from the heater matrix I think it is), they are very hot also.
Have had someone tell me that they think there’s a problem with coolant and once it goes to heater matrix its not keeping its heat when the blower blows it. Have additionally been told by another garage it could possibly be the head gasket is going!! Theyve said that theyve cleared airlocks and have topped up the coolant levels but heating only stays hot for 30 secs and then cools.
I dont have white smoke from exhaust, oil is not contaminated with coolant, neither is coolant with oil as have checked. Car seems to be running perfectly well and performance isnt affected. Car is not over heating either and gets to 90 degrees after 10-15mins of engine running. Please help. Only had car for 6months and cant afford another one!! Having block test done soon btw. Any ideas anyone?